For a polyolefin, particularly, an ethylene polymer, its molecular weight distribution is a considerably important property which gives an influence on both the processability of its molten substance and final mechanical properties. The application of a polyolefin therefore changes depending on its molecular weight distribution. There is a tendency that polyolefins of a wider molecular weight distribution are used for molded articles such as pipe, those of a medium distribution are used for fibers, tapes and the like, and those of a narrow distribution are used for injection-molded articles such as bottle caps, buckets and the like. In general, the molecular weight distribution of the ethylene-polymer largely depends on the properties of a solid catalyst component employed upon polymerization and it is necessary to choose a proper solid catalyst component according to the intended molecular weight distribution. As a catalyst permitting a wide molecular weight distribution, known are catalysts comprising a large variety of transition metal compounds in combination and solid catalysts having a specific surface area or porosity, as described, for example, in JP-B-52-37037 and JP-B-53-8588 ("JP-B" as used herein means an examined published Japanese patent publication) and JP-A-6-220117 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). For a narrow molecular weight distribution, on the other hand, a number of processes using a specific ether compound upon synthesis of a solid catalyst or upon polymerization are proposed, as described, for example, in JP-A-2-289604, JP-A-3-294302 and JP-A-3-294310. The above catalysts each serves to provide the intended molecular distribution, but has an unsatisfactory activity per transition metal because transition metals other than a titanium compound are used or an electron donor, such as an ether, which becomes a catalytic poison is added upon synthesis of a solid catalyst or upon polymerization. They are also insufficient from the viewpoint of the powder properties such as particle size and bulk density of the polymer. A solid catalyst component which contains an organomagnesium compound, Si--H bond-containing chlorosilane compound, alcohol and titanium compound as essential components is disclosed in JP-B-2-42366 in which, however, no suggestion is included concerning the controlling method of the molecular weight distribution of a polymer. In JP-B-5-7405, disclosed is a process for obtaining a polymer having a wide molecular weight distribution by using, as an olefin polymerization catalyst composed of a specific magnesium/titanium component, a catalyst in which a molar ratio of an alkoxy group/titanium in a reaction product is lower than a certain value. In the above process, however, activity per titanium metal is insufficient. In U.S. Pat. Nos. 4,159,965 and 4,471,066, disclosed is an olefin polymerization catalyst which can provide polymers having a molecular weight distribution over a wide range from narrow to wide molecular weight distribution. In this catalyst, the molecular weight distribution is controlled by using different transition metal components in combination. The activity per transition metal is however not enough for obtaining a polymer having a wide molecular weight distribution. Furthermore, upon preparation of a polyolefin, it is requested in recent years to properly prepare catalysts capable of providing different molecular weight distributions by using solid catalyst components prepared from the same raw materials and by the same preparation process with a view to reducing the manufacturing cost. Therefore, the above-described process using different transition metals in combination is disadvantageous from the viewpoint of the manufacturing cost.
As a process for preparing a polyolefin by polymerizing or copolymerizing an olefin in the presence of such a catalyst, commonly employed is so-called slurry polymerization in which polymerization is effected with a solid catalyst being suspended in a solution. In the case of slurry polymerization, however, a low-molecular weight polymer by-produced upon the polymerization reaction easily dissolves in a polymerization solvent and deposits on the wall of a reactor, which reduces the heat transfer coefficient and makes it difficult to stably carry out long-term continuous operation. Furthermore, the conventional solid catalyst component has a problem in its storage stability, more specifically, lowering in the polymerization activity when stored for a long period of time after preparation. Thus, the conventional solid catalyst component has a difficulty to be used in long-term continuous stable operation.